1. Field of the Invention
The present invention relates to a propagation time measuring device for measuring propagation delay times of signal transmission lines in a TDR (Time Domain Reflectometer) method.
2. Background Art
Description will be given with respect to a conventional device for measuring propagation delay times of signal transmission lines with reference to FIGS. 6-9c.
FIG. 6 is a block diagram showing the configuration of a conventional propagation delay time measuring device. FIG. 7 is a flow chart showing a propagation delay time measuring procedure carried out by the device shown in FIG. 6. FIGS. 8A-8C show signal waveforms which are detected in the operation of the system.
In FIG. 6, 8 designates a test signal generating circuit; 9 designating a measuring point at which a signal waveform is to be detected; 10 designates a signal transmission line; 11 designates a open terminal of the signal transmission line; 12 designates a time measuring circuit for measuring times when the voltage of the signal at the measuring point 9 becomes predetermined reference voltages; and 14 designates a calculation circuit for calculating a propagation delay time of the signal transmission line 10 based on the times measured by the time measuring circuit 12.
In FIGS. 8A-8C, 19 designates a waveform of a test signal which is generated by the test signal generating circuit 8 and has a maximum voltage Vh, and a minimum voltage 0 V, and a rise time Tr; 20 designates a signal waveform at the open terminal 11; and 21 designates a signal waveform at the measuring point 9.
Next, the operation of the conventional propagation delay time measuring device will be described according to the flow chart shown in FIG. 7.
In step S700, the test signal generating circuit 8 supplies the test signal 19 to the signal transmission line 10, and the time measuring circuit 12 measures a time T.sub.1 when the voltage of signal waveform 21 at the measuring point 9 becomes a reference voltage V.sub.2 in order to determine a time when the test signal is substantially supplied to the signal transmission line 10.
Next, in step S702, the time measuring circuit 12 measures a time T.sub.2 when the voltage of the signal waveform 21 at the time measuring point 9 becomes a reference voltage V.sub.3 in order to determine a time at which a reflected waveform from the open terminal 11 is returned to the measuring point 9.
Next, in step S704, the calculation circuit 14 calculates a difference T.sub.2 -T.sub.1 as a reciprocal propagation delay time T.sub.rfl for reciprocating the test signal through the signal transmission line 10.
Next, in step S706, the calculation circuit 14 divides the reciprocal propagation delay time T.sub.rfl by 2 to determine a propagation delay time T.sub.pd for transmitting the test signal from the measuring point 9 to the open terminal 11 through the signal transmission line 10.
The conventional device assumes an ideal condition in which the test signal propagates through the transmission line with no attenuation. In actual signal transmission, however, the test signal is attenuated when propagating through the signal transmission line. Therefore, the conventional device has a problem in that the large measurement error due to the attenuation of the test signal is generated in the measurement of the propagation delay time and it is difficult to accurately determine the propagation delay time.
Next, description will be given with respect to this problem referring to FIGS. 9A-9C. FIGS. 9A-9C show signal waveforms which are obtained in the measuring device. In FIGS. 9A-9C, 22 designates a signal waveform which is detected at the open terminal 11; and 23 designates a signal waveform which is detected at the measuring point 9. In the case shown in FIGS. 9A-9C, the signal is transmitted through the signal transmission line 10 with an attenuation. Therefore, the slopes of the signal waveforms 22 and 23 are curved due to the attenuation.
In this case, the reciprocal propagation delay time T.sub.rfl does not equal twice the propagation delay time T.sub.pd. Thus, the propagation delay time T.sub.pd is measured with a measurement error T.sub.error. For example, the measurement error T.sub.error has a value of several tens of ps when the propagation delay time to be measured is several ns.